Injection and extrusion moulding are two well-known technologies for manufacturing items from polymer, glass, ceramic or even metal and have been known and studied for decades.
In injection moulding, raw material in the form of granules is fed into a granulate inlet and from there to a barrel where it is transformed into a liquid melt under the influence of heat and mechanical forces. Usually, the mechanical force on the granules is exerted by a screw which is rotatable inside and movable within the barrel. Once a certain mass of molten granules has gathered in the barrel, the screw is retracted from the barrel, due the increasing volume of molten mass. In the next step, the screw is pushed back into its original position pushing the mass of raw molten material out of the barrel under pressure. In order to release the pressurized molten raw material into the mould, an injection needle closing the access to an injection mould is opened and the pressurized molten raw material is released into the mould. Usually, more than one injection needle is present, such that the molten mass may enter the mould from several locations. Usually, several moulds are present in such an injection moulding apparatus as well. After cooling down in the mould, the molten mass hardens into a solid material which then may be pushed or blown out of the mould, making space for injection moulding of yet another solid item. The entire process is then repeated.
Usually, any material that can be transformed into liquid phase may be used as raw material for injection moulding, such as metal, glass, ceramics and various types of polymers. Extrusion moulding has a lot in common with injection moulding, since in extrusion moulding a raw material is melted into liquid form by means of a rotation screw and heat. However, the main difference is that the molten raw material is continuously fed out of the barrel into the mould through a two-dimensional die. Another difference between the two methods is that in extrusion moulding the extruder screw is not retracted from the barrel before being released into the barrel
Several problems arise when using injection or extrusion moulding continuously in an industrial process.
Firstly, traditional injection and extrusion moulding processes have an inherent inertia due to the movement of the extruding screw which needs time to speed up or slow down its movement. As a consequence, liquid pressure in such traditional systems cannot be controlled with the desired speed. This makes them less suitable for injection moulding methods where speed is essential.
Secondly, existing solutions are dependent on the presence of an accumulator as a raw material melt buffer. Traditional accumulators are at risk of leaking the liquid raw material and “burning” it. Moreover, problems may arise when raw material of one colour is to be exchanged for another material of a different colour. Here, a so called dead-end problem occurs, for example in the accumulator, meaning that remains of the old raw material, which for example may be white are mixed with the new raw material which may be black resulting in injection moulded objects that are greyish. This is, of course, undesirable in end products. Thus, there is a desire to provide an improved injection moulding method and apparatus which solves at least some of the problems associated with known technology.